Method for Processing Ore to Minimize the Production of Acidic Wastes

ABSTRACT

A method for processing ore using carbonates so as to minimize the production of acidic wastes. The method includes adding the carbonates to the ore and after the carbonates have been added to the ore, substantially jointly grinding the ore and the carbonates to produce a treated ore mixture. Substantially jointly grinding the ore and the carbonates substantially mixes and homogenizes the ore and the carbonates so that acids subsequently created within the treated ore mixture are substantially neutralized within the treated ore mixture by the carbonates.

FIELD OF THE INVENTION

The present invention relates to the general field of mining and is particularly concerned with a method for processing ore to minimize the production of acidic wastes.

BACKGROUND OF THE INVENTION

It is well-known in the art that mining and storing sulphide-bearing materials such as mine spoils, metal ores, coal seams and tailings, among others, often results in the generation of acidic wastes which, in turn, are detrimental to the environment. Indeed, acidic wastes adversely affect fauna and flora. Yet, furthermore, when an acid generation potential of an ore exceeds the neutralization capacity of the ore, sulphuric acid is often generated. The sulphuric acid may then be dissolved in water to produce a sulphuric acid solution. Such sulphuric acid solutions exert an adverse influence on concrete structures such as bridges and dams. Furthermore, acids and heavy metals such as iron and arsenic that may be dissolved by the acids degrade water quality, and may exterminate shellfishes and fish alike.

The oxidation of sulphur-bearing compounds such as mineral pyrite or the like indeed produces acidic solutions. The chemical reactions involve the oxidation of sulphur-bearing compounds in the presence of oxygen and water to form iron hydroxide and sulphuric acid. The acidic solutions are commonly called acid rock drainage or acid mine drainage. These acidic solutions dissolve and mobilize several metals, in particular, iron and manganese, which, under neutral or basic conditions, remain relatively insoluble.

In the prior art, problems associated with acid mine drainage have been addressed mainly by concentrating the acid generating sulphides and storing them into a body of water such as a basin or lake. The flooding or immersion of such sulphides prevents the contact with air and, hence, prevents the sulphides from being oxidized. Although somewhat useful, this prior art method suffers from numerous drawbacks.

Indeed, in certain settings, the immersion of sulphides requires the creation of a setting pond and maintenance thereof in order to prevent drainage of the pond which if such is the case would eventually lead to sulphides oxidization and, hence, acid generation. Furthermore, with some ores, it is simply not yet feasible to recover enough sulphides to turn the waste into non-acid generating material.

Against this background, there exists a need for an improved method for processing ore to minimize the production of acidic wastes. It is an object of the present invention to provide such a method for processing ore to minimize the production of acidic wastes.

SUMMARY OF THE INVENTION

In a first broad aspect, the invention provides a method for processing ore using carbonates so as to minimize the production of acidic wastes. The method includes adding the carbonates to the ore and, after the carbonates have been added to the ore, substantially jointly grinding the ore and the carbonates to produce a treated ore mixture. Substantially jointly grinding the ore and the carbonates substantially mixes and homogenizes the ore and the carbonates so that acids subsequently created within the treated ore mixture are substantially neutralized within the treated ore mixture by the carbonates.

Advantages of the present invention include that the proposed method allows for the production of non-acid generating mine wastes at relatively low costs. Also, the method is relatively easily incorporated to existing ore treatment processes.

Furthermore, the proposed method addresses the root of the problem associated with the creation of acidic wastes and, hence, is relatively efficient. In addition, the resulting mine wastes may be stored in conditions that require only minimal or no maintenance. Therefore, this method allows for an inherently relatively safe manner of storing the wastes that is not affected by socio-economic disturbances such as armed conflicts and bankruptcies that would prevent a necessary maintenance of a waste containing site.

In some embodiments of the invention, the carbonates are added in the form of minerals that are advantageously mined in proximity to an ore processing facility, thereby improving the cost-effectiveness of the proposed method.

It has been found that the treated ore mixture is relatively stable and can undergo most of commonly employed ore processing steps without losing its neutralizing properties.

In another broad aspect, the invention provides a method for processing ore using carbonates so as to minimize the production of acidic wastes. The method includes grinding the carbonates and the ore so that the carbonates and the ore have a substantially similar granulometry and mixing the ground carbonates and the ground ore to produce a treated ore mixture. Mixing the carbonates and the ore produces a treated ore mixture such that acids subsequently created within the treated ore mixture are substantially neutralized within the treated ore mixture by the carbonates.

Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings, the only FIGURE, in a flow chart, illustrates a method for processing ore using carbonates so as to minimize the production of acidic wastes in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

The proposed method is based on an observation that the difference between acid-generating ores and non-acid generating ores often resides in that the former does not contain enough naturally occurring carbonates to consume all of the acid that may be produced. The amount of generated acid is mainly linked to the difference between the quantity of acid that sulphides contained in the ore can generate and the quantity acid the carbonates contained in the ore can neutralize.

Hence, the proposed method relies on adding carbonates such as, for example, dolomite, calcite, a combination of dolomite and calcite, or any other suitable carbonates to an ore. For example, and non-limitingly, the carbonates are added to a grinding circuit of a processing plant in a predetermined proportion.

The use of carbonates is in contrast to the long-standing tradition in the mining industry of using hydroxide products for neutralizing acids. Indeed, in the mining industry, the most commonly used neutralization products are typically quick lime and dead lime (CaO or Ca(OH₂). Although these products are powerful acid neutralizers, they are relatively easily lixiviated by rain water.

It was found that surprisingly, after being added to the ore, the carbonates remain mixed with the ore during most processes currently performed on such ores. Also, in embodiments of the invention wherein the carbonates are substantially water-insoluble, the carbonates may remain contained within the ore for relatively long period of time even when the ore is stored exposed to rain.

The reader skilled in the art will readily appreciate that it seems counter-intuitive to add a neutralizing substance to the ore before the ore is processed to extract some components of the ore, such as for example metals, as this increases the quantity of ore to process. However, it was found that the additional cost of processing a larger quantity of ore has a potential to offset the costs of maintaining a waste containing site when the neutralizing substance is not used in this manner.

The above suggests a method 100 for processing ore using carbonates so as to minimize the production of acidic wastes illustrated in FIG. 1. The method 100 starts at step 105. At step 110, the ore is crushed. Then, at step 115, an acid generating potential and a neutralizing potential of the ore are assessed. Subsequently, at step 120, carbonates are added to the ore and the carbonates and the ore are substantially jointly ground at step 125 to produce a treated ore mixture. Substantially jointly grinding the ore and the carbonates substantially mixes and homogenizes the ore and the carbonates so that acids subsequently created within the treated ore mixture are substantially neutralized within the treated ore mixture by the carbonates. The method 100 then ends at step 130. After the method 100 has been performed, the treated ore mixture may be processed substantially similarly to the manner in which the ore is conventionally processed.

Step 115 of assessing the acid generating potential and neutralizing potential of the ore may be performed at any suitable stage. In some embodiments of the invention, step 115 is omitted and the carbonates are added to the ore in a predetermined quantity. In other embodiments of the invention, the neutralizing potential of the ore is not assessed and only the acid generating potential of the ore is assessed.

At step 120, the carbonates are added to the ore. In some embodiments of the invention, the carbonates are substantially insoluble in water, which weather proofs the treated ore mixture. For example the carbonates are added in the form of dolomite, calcite, a combination of calcite and dolomite or any other suitable carbonates. Such substances are indeed sometimes found in the proximity of ore processing plants, which therefore provides a relatively inexpensive source of carbonates. In other embodiments of the invention, the carbonates are at least partially soluble in water.

In some embodiments of the invention, the carbonates are added to the ore in an amount having an acid neutralizing potential sufficient for neutralizing at least the acid generating potential of the ore so as to ensure that there is at least a potential to neutralize all the acid that the ore may produce. For example, the carbonates are added to the ore in an amount having an acid neutralizing potential sufficient for neutralizing about twice the acid generating potential of the ore. Such a quantity of added ore has been found useful as it provides a safety margin in case some of the carbonates become unavailable for neutralizing acids generated within the ore, while remaining relatively inexpensive.

In some embodiments of the invention, the carbonates are added in a quantity considering only the acid generating potential of the ore. In these embodiments, there is no need to assess the neutralizing potential of the ore. In other embodiments of the invention, the carbonates are added in a quantity sufficient to complement the naturally occurring neutralizing potential of the ore.

It has been found that carbonates are efficient in neutralizing sulphuric acid produced in ore including sulphites. However, the treatment of ores in which any other substances produce acids is also within the scope of the invention.

At step 125, in some embodiments of the invention, the ore and the carbonates are ground in a manner such as to obtain a treated ore mixture in which the ore and the carbonates have a substantially similar granulometry. This facilitates the homogenisation and mixing of the ore and carbonates.

While in the method 100 the carbonates and the ore are ground substantially jointly, it is within the scope of the invention to mix the carbonates and the ore in any other suitable manner to produce the treated ore mixture. In some embodiments of the invention, the carbonates and the ore are ground so that the carbonates and the ore have a substantially similar granulometry and afterwards mixed. For example, the carbonates and the ore are ground separately from each other before being mixed together.

In embodiments of the invention wherein the ore and the carbonates are ground substantially jointly, the carbonates may be added at any stage before being ground, such as for example during the crushing step 110.

While the method 100 includes a step 110 of crushing the ore, in alternative embodiment of the invention, the ore processed has a granulometry that make step 110 unnecessary and this step may then be omitted.

When carbonates are used, acidic products such as acid generated by mine waste are the main substances that can make them react. When the minerals produce acid, it is entrapped by the carbonates, which will turn acid into nonpolluting gypsum.

Typically, the carbonates are added in the grinding circuit of the processing plant in such a manner that the granular size of the added carbonates is substantially similar to that of the carbonates contained in the ore. Hence, the addition of the carbonates forms a homogeneous mixture wherein indigenous and added carbonates are substantially indistinguishable.

It should be understood that although the term “carbonate” is used throughout the text, the reader skilled in the art will readily understand that calcium carbonate (calcite), calcium carbonate-magnesium (dolomite) or any other suitable substance may be used without departing from the scope of the present invention.

Carbonates can also be blended into barren ore when sent to a barren paddock. Again, the carbonates and barren ore must be similar in granular size.

The proposed method acts on the quantity of sulphuric acid generated by sulphites contained in the rejects by turning acid dissolved into water and sulphur in the form of sulphite into gypsum, as shown in the following equation:

for calcite:

CaCO₃+H₂SO₄=CaSO₄+H₂O+CO₂

CaCO₃+ZnSO₄=CaSO₄+ZnCO₃

for dolomite:

CaMg(CO₃)²+H₂SO₄=CaSO₄+MgCO₃+2H₂O+CO₂

The reader skilled in the art will note that calcium sulphide is insoluble. Hence, it will stay still in wastes and will not damage the environment. Similarly, metal carbonates such as zinc carbonate, will also remain still in wastes.

Although the present invention has been described hereinabove by way of preferred embodiments thereof, it can be modified, without departing from the spirit and nature of the subject invention as defined in the appended claims. 

What is claimed is:
 1. A method for processing ore using carbonates so as to minimize the production of acidic wastes, said method comprising: adding said carbonates to said ore; and after said carbonates have been added to said ore, substantially jointly grinding said ore and said carbonates to produce a treated ore mixture; whereby substantially jointly grinding said ore and said carbonates substantially mixes and homogenizes said ore and said carbonates so that acids subsequently created within said treated ore mixture are substantially neutralized within said treated ore mixture by said carbonates.
 2. A method as defined in claim 1, wherein adding said carbonates to said ore includes adding carbonates which are substantially insoluble in water to said ore.
 3. A method as defined in claim 1, wherein adding said carbonates to said ore includes adding dolomite to said ore.
 4. A method as defined in claim 1, wherein adding said carbonates to said ore includes adding calcite to said ore.
 5. A method as defined in claim 1, further comprising assessing an acid generating potential of said ore, said carbonates being added to said ore in an amount having an acid neutralizing potential sufficient for neutralizing at least said acid generating potential of said ore.
 6. A method as defined in claim 5, wherein said carbonates are added to said ore in an amount having an acid neutralizing potential sufficient for neutralizing about twice said acid generating potential of said ore.
 7. A method as defined in claim 1, further comprising assessing an acid generating potential of said ore and an acid neutralizing potential of said ore, said carbonates being added to said ore in an amount such that an acid neutralizing potentials of said carbonates and said acid neutralizing potential of said ore are together sufficient for neutralizing at least said acid generating potential of said ore.
 8. A method as defined in claim 7, wherein said carbonates are added to said ore in an amount such that said acid neutralizing potentials of said carbonates and said acid neutralizing potential of said ore are together sufficient for neutralizing about twice said acid generating potential of said ore.
 9. A method as defined in claim 1, wherein said ore and said carbonates are ground in a manner such as to obtain a treated ore mixture in which said ore and said carbonates have a substantially similar granulometry.
 10. A method as defined in claim 1, wherein said ore includes sulphites having a potential to generate sulphuric acid.
 11. A method as defined in claim 1, further comprising crushing said ore before grinding said ore.
 12. A method as defined in claim 11, wherein said carbonates are added to said ore before said ore is crushed, said ore and said carbonated being thereby crushed jointly.
 13. A method for processing ore using carbonates so as to minimize the production of acidic wastes, said method comprising: grinding said carbonates and said ore so that said carbonates and said ore have a substantially similar granulometry; mixing said ground carbonates and said ground ore to produce a treated ore mixture; whereby mixing said carbonates and said ore produces a treated ore mixture such that acids subsequently created within said treated ore mixture are substantially neutralized within said treated ore mixture by said carbonates.
 14. A method as defined in claim 13, wherein said carbonates and said ore are ground separately from each other before being mixed together. 